Fixed wireless loop (FWL) communications systems support distribution of data and voice transmission. Such systems are usually segmented into “cells.” A base-station antenna located within each cell transmits signals to, and receives signals from, a plurality of terminals or peripheral stations also located within the cell. The cell need not be contiguous; the base station of one cell may service a select region or regions within the nominal boundaries of a nearby cell as geography or other factors dictate. The large number of transmitting sources present in FWL systems create a potential for a significant amount of interference with the communication between any particular base station antenna and terminal. Such interference can be caused by other transmitters within the cell, or in other cells.
FWL systems typically utilize methods of frequency division multiplexing (FDM), time division multiplexing (TDM) or code division multiplexing access (CDMA)) to maximize system capacity and mitigate interference. Presently, it is widely believed that CDMA-based FWL systems are superior to TDM and FDM systems in terms of achieveable capacity. The main reason for this belief is that TDM and FDM are limited to high frequency reuse factors, typically about seven, while the frequency reuse factor for CDMA can be set to one. Sectorized antennas further increase the perceived advantage of CDMA, because it is typically considered impractical to lower the reuse factor of TDM-based systems even when sectorized base station antennas are available.
CDMA-based systems possess a limitation, however, that is not shared by TDM-based systems. In particular, in CDMA-based systems, the base station antenna continuously illuminates all the terminals within a cell or sector. In TDM-based systems, the base station antenna for a particular cell illuminates only those terminals that are active during a particular time slot. The potential therefore exists for TDM-based systems to collect less interference from other emitters and to generate less interference to other receivers.